Hi guys,
I thought I'd drop a line as I'd mentioned a while back I was going to play with some digital audio sender devices in the 2.4GHz band for STL ideas. Since digital modulated devices are allowed 4 Watts after the antenna, not 1W like analogue.
I bought a couple of cheap second hand DAS-100 units which has www.elansat.com modules in them off trademe once. The module I use are a small self contained 2.4GHz FSK transmitter, D/A and A/D TX/RX respectively. The one I have encodes audio in to 16-bit 44,100Hz stereo, 95dB SNR and range but unbalanced input. It's not a video sender, but rather a dedicated digital audio sender. They take audio up to around +4dBu (assume one drives them with a sound card up to 0 with 3-4 headroom for quantization errors).
I had the privilledge of outputting one of these units in to a 21-24dB antenna (never have the TX antenna going with 2.4GHz close to your body) at the TX and RX end, going on the default 10mW output, over 3KM's non-line of site in a city from single level roof top with antennas about 2-3 meters above roof line. While the signal unlocked now and then at 10mW (and considering the antennas can't see each other), the sound on them is terrific. A quick test with an older DSE 500mW Wi-Fi booster made it rock solid. However I'm looking at testing a 100-200mW selectable Edimax booster soon for obvious RF power reasons. I think the Hyper Technologies and Pacific Wireless 12dB mini directional square antennas with high loss coax would be ideal.
So my eventual experiment results are, even on 10mW TX output in to a 21+dB antenna with another one on the receive end (probably about 8-10dB loss in cable, amazingly impressed me mucho grande! The great part though.... the modules also use high quality "forward error correction", I believe with this one (I lost my book on it) is 1/2, bit for bit error correction. This compared to freeview satellite delivery only being around 1 error correcting bit for every 3 (3/4). The unit I tested and modified (just the on-board antenna and audio inputs) has 8 channels to choose from.
The newer modules and chips do away with selectable channels in favour of frequency hoping instead with 48KHz A/D's and D/A's.
Anyway errors weren't a problem working along around a high..ish powered wireless ISP and numerous other WLANs in the area accept on less than 10mW TX power. On the default channel the microwave at home caused the RX to unlock but I know it works well on anything above the default channel at home (I'm awaiting frequencies from manufacturer so I can choose and map out better one). The only draw back is I haven't figured out how to lock the RX on a channel. The channels are button selectable on the TX and the RX auto tunes with diversity antennas (all on board by default). Autotune not being a good idea although not many people in NZ have "digital" audio senders.
They are used in digital wireless audio headphones. The other boards they do, have intergrated USB sound cards in the TX although the unit I tested had 3.5mm stereo jacks with a D/A in the TX and a A/D obviously in the receiver. (I promptly replaced the 3.5mm sockets as they didn't take to many plug ins and plug outs before getting scratchy).
All in all, while at one stage I liked SPDIF over analogue video senders (unfortunatelly without error correction and limited to 1W E.I.R.P), I'm now a big fan of forward error corrected digital audio senders and modifying the internal antennas to go to a booster in front of them.
I plan on tidying up my little mucking around and putting everything in steel project cases with selectable output power and hope to test the TX with an Edimax booster. As soon as I can lay my hands on the FHSS (frequency hoping spread spectrum) modules I'll get around to posting more test results and I'm sure I'll be even more impressed with it's stability.
I'm gathering more information about where the best place to get the boards or newer boards from to NZ is. However I haven't had a look at the devices to see if the intergrated antennas are easily tapped on the new ones. Will let you know again one day how more experiments like this continue.
Gavin.
Offline 
Send Email